1. Field of the Invention
This invention pertains in general to switchgear and other switching equipment which use hermetically sealed interrupters employing insulation having a higher electrical resistance than air and more particularly to compact switchgear that employ modular components to reduce the overall size of the system.
2. Background Information
Circuit breakers provide protection for electrical systems from electrical fault conditions such as current overloads, short circuits, and abnormal level voltage conditions. Typically, circuit breakers include a spring powered operating mechanism which opens electrical contacts to interrupt the current through the conductors on an electrical system in response to abnormal conditions. In particular, vacuum circuit interrupter apparatus have been known which include separable main contacts disposed within an insulating housing. Generally, one of the contacts is fixed relative to both the housing and to an external electrical conductor which is interconnected with the circuit to be controlled by the circuit interrupter. The other contact is moveable. In the case of a vacuum circuit interrupter, the moveable contact assembly usually comprises a stem of circular cross-section having the contact at one end enclosed within a vacuum chamber and a driving mechanism at the other end which is external to the vacuum chamber. An operating rod assembly comprising a push rod, which is fastened to the end of the stem opposite the moveable contact, and a driving mechanism provide the motive force to move the moveable contact into or out of engagement with the fixed contact.
The operating rod assembly is operatively connected to a latchable operating mechanism which is responsive to an abnormal current condition. When an abnormal condition is reached, the latchable operating mechanism becomes unlatched which causes the operating rod to move to the open position. The motion of the operating rod, in turn, causes the contact bell crank to rotate and, as discussed above, this controls motion of the moveable contact.
Compression springs are provided in connection with the operating rod assembly in order to be able to separate the moveable contact from the fixed contact and to assure the necessary force so that the contacts will not accidentally open under inappropriate conditions. In addition, when appropriate circumstances requiring interruption of the circuit do arise, an adequate force is needed to open the contacts with sufficient speed. If the contacts do not open quickly, there is a risk of the contacts welding together and failure to interrupt the current.
Vacuum interrupters or reclosures are typically used, for instance, to reliably interrupt medium voltage ac currents, and in the case of vacuum interrupters also high voltage ac currents of several thousands of amperes or more. Reclosures include interrupters that are designed to typically operate in a range of current measured in amperes, rather than kiloamperes, and automatically reclose after a given delay, and for a given number of times, in an attempt to automatically reestablish the circuit. However, from the standpoint of this invention the principles discussed herein apply equally well to both, as well as to other load break switches, circuit breakers and switch disconnects.
Typically, in accordance with prior art practices, one vacuum interrupter is provided for each phase of a multi-phase circuit and the interrupters for the several phases are actuated simultaneously by a common latchable operating mechanism. This does not permit point-on-wave switching among the multiple phases and can give rise to voltage transient and uneven contact wear.
Medium size switchgear employing vacuum interrupters typically have a front low voltage portion just to the rear of the front panel which typically includes the breaker controls. The low voltage portion is electrically insulated, through the use of standoffs, from high voltage portions which include the vacuum interrupters. The standoffs permit sufficient spacing of the conducting components so that air within the housing can be an effective insulator to isolate the low voltage portions of the switchgear from the high voltage components. However, this spacing also contributes to the size of the units.
More recently attempts have been made to reduce the size of these units by hermetically sealing them and filling them with a gaseous insulator having a higher dielectric capacity than air, such as SF.sub.6. However, this creates a number of environmental concerns.
Accordingly, an improved medium voltage switchgear design is desired that reduces the overall size and enhances the serviceability of the unit.